The present invention relates to a ring laser gyroscope. More particularly it relates to a method for fabricating a readout apparatus for such a ring laser gyroscope.
Ring laser gyros are well known and are particularly described in U.S. Pat. No. 3,373,650, issued to Killpatrick, and U.S. Pat. No. 3,390,606, issued to Podgorski. Ring laser gyros of the type referred to commonly utilize a block of material that is dimensionally stable, both thermally and mechanically. The block usually includes a plurality of interconnected gas-containing tunnels or passages which form a closed-loop path in the shape of a triangle, a rectangle, or any polygonal path. At each intersection of a pair of interconnected tunnels is a mirror mounted on the block. This arrangement of mirrors and interconnected tunnels forms an optical closed-loop path. Further, at least one anode and one cathode are each mounted on the block and in communication with the gas. Each of the components, including the mirrors, anode, and cathode, must be sealed to the block to form a gas-tight seal. The block is usually filled with a lasing gas such as a mixture of helium and neon. A sufficiently large electrical potential is applied between the anode and cathode to cause a discharge current therebetween which results in the production of a pair of counter-propagating laser beams within the block.
In order to obtain useful information from a ring laser gyro of the type described above, a small percentage of each of the counter-propagating laser beams is allowed to pass through one of the mirrors, known as the output mirror. The light beams passing through the output mirror are commonly passed through a prism which combines the beams at slightly different angles and thereby creates a readout interference fringe pattern. When the gyro is rotated about its input axis, the counter-propagating beam frequencies change slightly, one increasing and the other decreasing in proportion to the rotation rate of the gyro. The frequency difference between the frequencies of the beams results in a beat frequency which is indicated by a rate of movement of the fringe pattern across the photodetectors, as is well known.
The resulting interference fringe pattern which exits from the prism is commonly directed to impinge upon a pair of photodetectors. Generally, in the art of ring laser gyros, the readout apparatus requires the pair of photodetectors to be separated by one quarter of the fringe spacing of the readout interference fringe pattern. With the photodetectors separated by the aforesaid spacing, the photodetector output signals will be in phase quadrature in response to a moving interference fringe pattern, as is well known.
In U.S. Pat. No. 4,152,072 issued to Hutchings, a laser gyro readout interference fringe pattern is projected onto a mask before impinging upon a pair of photodetectors. The mask, as taught by the aforementioned patent, consists of parallel alternating opaque and transparent regions, or stripes, which are in parallel with the fringes of the readout interference fringe pattern. The mask stripes of Hutchings are spaced at slightly different intervals than the impinging readout interference fringe pattern. As the fringe pattern moves across the mask, the photodetectors provide output signals which are out of phase relative to each other, indicative of the beat signal.
In turn, the fringes passing by either photodetector are counted for indication of rotation angle, and the pair of out of phase photodetector signals are used to determine direction.
Alternatively, U.S. Pat. No. 4,676,643, issued to Vescial, teaches another mask readout system for obtaining a pair of out of phase photodetector signals. In Vescial, the mask consists of parallel alternating opaque and transparent stripes having the same periodicity as the readout interference fringe pattern. The mask is positioned in front of a pair photodetectors. The mask is oriented relative to the readout interference fringe pattern such that there is a predetermined angle between the stripes of the mask and the fringes of the readout interference fringe pattern.
As taught by Vescial, by virtue of the readout interference fringe pattern passing through the "tilted" mask, a light spot is created on the photodetector side of the mask. The resulting light spot exhibits a Moire fringe pattern characteristic. In turn, photodetectors which are spaced by one quarter of the Moire fringe pattern periodicity respond to the Moire interference fringe pattern. The photodetectors then provide output signals which ar out of phase relative to each other, indicative of the beat signal. The Moire fringes passing by either photodetector are counted for indication of rotation angle, and the pair of out of phase photodetector signals are used to determine direction in the usual manner.
Generally, the spacing of the parallel stripes of the mask in either the Hutchings patent or the Vescial patent is chosen with respect to the fringe spacing of the readout interference fringe pattern.
Due to variances in angular alignment of the output mirrors between "identical" gyros, there is great variation in orientation and spatial frequency between their respective fringe patterns. Consequently, since mask fabrication depends upon fringe spacing, identical gyros require individual mask fabrication.